The present invention relates to processes for forming barrier layers on metal surfaces. It finds particular application in conjunction with forming barrier layers on titanium and aluminum containing substrates, particularly titanium aluminides, which resist oxidation, resist corrosion, resist wear and abrasion, and resist corrosive media.
Titanium aluminide is currently being investigated to replace superalloys for use in aircraft turbine engines and aircraft structures. Titanium aluminide is about half the density of superalloys of comparable strength, so a large reduction in aircraft weight is possible. The titanium aluminide alone is quite brittle, but workers have been able to add other elements to reduce this brittleness. A remaining development problem is that the oxidation resistance of these titanium aluminide compounds is lower than desired at elevated temperature. Therefore, a key factor in increasing the maximum use temperature is the enhancement of oxidation resistance while maintaining creep and strength performance.
Previous attempts to develop a protective coating have resulted in coatings that are unstable or tend to peel off. If a titanium aluminide substrate is oxidized in air or oxygen at high temperature, as is conventionally done, Al2O3 and TiO2 are formed. These two oxides have different structures and are immiscible in each other. As such, the mixed oxide is porous and weakly bonded to the substrate. Therefore, they are subject to spallation from the substrate. As such, the oxides are not an effective oxygen barrier. This is, they do not prevent the diffusion of oxygen into the substrate and the reaction of oxygen with aluminum, titanium, and other elements below the surface.
The present invention relates to a new and improved technique for forming a strongly-bonded surface barrier and overall mixed-oxide protective coating for titanium aluminide substrates, which overcomes the above-referenced problem, and to the structures produced by such a technique.